The core method for determining if a hot runner system is overheated is through metallographic analysis to confirm the presence of irreversible damage characteristics such as oxides distributed along grain boundaries, remelted spheres, or "candy-like" cracks. Even without macroscopic cracks, abnormal microstructure is sufficient to determine overheating.
1. Metallographic Microscopic Analysis: The Most Reliable Judgment Method
Characteristics Overheating Signs Normal Structure Comparison
Grain Boundary Oxide Network Continuous or discontinuous black oxides (such as Cr₂O₃) form at grain boundaries; EDS analysis shows oxygen enrichment along grain boundaries. Clear grain boundaries, no oxides, uniform element distribution.
Intergranular Cracks (Candy-like Fracture) Cracks strictly extend along grain boundaries, appearing "stone-like" or "candy-like," without plastic deformation. No cracks, occasional transgranular cracks are due to processing.
Remelting Balls Rounded droplet-like solidified structures (<5μm) appear at grain boundary junctions, indicating localized melting. No remelting traces, precipitated phases are regularly distributed.
Abnormal Grain Coarsening Significantly coarse grains (H13 steel > ASTM 5 grade), uneven in size. Fine and uniform grains, conforming to material standards.
Judgment Basis: Based on GB/T 6394 Standard rating: The presence of any of the above characteristics confirms overheating and necessitates scrapping.
2. Auxiliary detection methods to improve accuracy:
EDS energy dispersive spectroscopy: Surface or line scanning of suspicious grain boundaries. If enrichment of elements such as O, Cr, and Fe is detected, it confirms true oxidation rather than surface contamination.
High-magnification microscopy (800×~1000×): Identifies submicron-level oxides and early voids, avoiding missed detections.
Microhardness testing: Overheated areas often exhibit both softening zones (↓≥15 HRC) and hard areas, reflecting severe microstructural heterogeneity.
Practical tip: For workpieces that have undergone overheating (>1150°C), mandatory metallographic sampling inspection should be performed even if the appearance is normal.
3. Macroscopic Preliminary Assessment and Service History-Assisted Identification
Methods Observable Features
Naked Eye/Magnifying Glass Observation: Surface discoloration (deep blue, purplish-black), blistering, "prickly heat"-like bumps
Penetration Testing (PT): Shows fluorescent or colored traces of surface-opening cracks
Magnetic Particle Testing (MT): Forms clear magnetic traces under a magnetic field, indicating crack location (applicable to ferromagnetic materials)
Heating Record Verification: Exceeded the safe heating temperature (1150°C) of H13 steel or held at that temperature for an excessively long time.
Risk Warning: "No cracks ≠ safe." Subsurface oxidation and grain boundary voids can exist without macroscopic signs.
4. Correct Handling Principles: Zero Tolerance Judgment
If any overheating characteristic is found, the device must be scrapped immediately; repair or downgrading is prohibited.
Replace with new parts: Prioritize overheat-resistant materials such as PM-H13 and H13+Ti.
Trace the root cause: Check for inaccurate temperature control systems, thermocouple drift, and PID parameters malfunction.
Safety Closed Loop: Establish a "detection → judgment → scrapping → traceability" mechanism to prevent similar accidents from recurring.
